1. Field of the Invention
The present invention relates to an electro-optic (hereinafter, abbreviated as EO) sampling probe, which is used for observing an waveform of a test signal based on a change of the polarization state of a light pulse caused when the light pulse generated based on the timing signal is admitted into an electro-optic crystal which is coupled with an electric field generated by the test signal, and particularly relates to an electro-optic sampling probe provided with an improved probe optical system.
2. Background Art
First, the principle of the electro-optic (hereinafter, abbreviated as EO) sampling measurement will be described with reference to FIGS. 4 and 5.
In FIG. 4, the reference numeral 10 denotes a test object such as a semiconductor IC wafer etc., which is connected to an electric signal probe 33, and to which electric signals from a signal generator 34 are supplied. The test object 10 is operated based on the electric signals.
The reference numeral 1 denotes the electro-optic crystal (hereinafter, called the EO crystal). The numeral 5 denotes an object glass for-condensing light incident on the EO crystal 1. The numeral 6 denotes an observing optical system comprising a dichroic mirror 29, a half mirror 23, and an observing light source 26. The numeral 22 denotes an EO sampling optical system comprising a photo-diode (not shown) and a light isolator (not shown), and a fiber collimator 30 is disposed at one end of the EO sampling optical system 22. The numeral 21 denotes a pulse laser light source for supplying light to the fiber collimator 30.
The reference numeral 27 denotes an infrared camera for localization of the laser light in order to condense the light onto a wiring pattern on the test object 10, and the alignment is confirmed by use of an image monitor 28. The numeral 35 is an XY stage for aligning the test object. The numeral 24 denotes a differential amplifier for differentially amplifying an output signal obtained when the change of the polarization state of the laser light is converted into an electric signal by means of the EO sampling optical system 22. The numeral 25 denotes a waveform indicator for indicating the electric signal.
Next, referring to FIG. 4, the path of the laser light emitted from the pulse laser source 21 is described.
First, the laser light admitted into the EO sampling optical system 22 through an optical fiber from the pulse laser source 21 is collimated by the fiber collimator 30 and admitted into the observing optical system 6 after rectilinearly propagating in the EO sampling optical system 22. The laser light further rectilinearly propagates in the observing optical system 6 and is then condensed on the test object 10 through the EO crystal 1 by the objective lens 5.
The wavelength used in this sampling optical measurement is 1550 nm. The dichroic mirror 29 is characterized by the transmittance and reflectance for the light of the wavelength of 1550 nm being 95% and 5%, respectively, and the transmittance and reflectance for a light having an wavelength of 1300 nm is 20% and 80%, respectively. Accordingly, 95% of the laser light emitted by the pulse laser source 21 is transmitted and irradiated on the test object 10.
The light beam reflected by the test object is collimated into a parallel beam again by the objective lens 5, and returns to the EO sampling optical system 22 through the same light path as that for entering the photodiode (not shown) in the EO sampling optical system.
An explanation is described hereinafter of the path of light emitted by the observing light source 26 in the case of executing alignment of the test object by use of the observing light source 26 and the infrared camera 27. The lamp used for the observing light source 26 is a halogen lamp which emits light having wavelengths ranging from 400 to 1650 nm.
The light beam, emitted from the observing light source 26, passes the half mirror 23, advances rectilinearly, and irradiates the test object after being turned at a right angle by the dichroic mirror 29. The half mirror 23 used in this optical system is a mirror, in which both transmittance and reflectance are equal to 50%.
The infrared camera 27 takes a photograph of a part of the test object, irradiated by the observation light source 26 in the view field of the objective lens 5, and the obtained infrared image is displayed on the monitor 28.
An operator moves the XY stage 35 slowly while watching the image displayed on the monitor 28, and adjusts the position of the wiring to be tested on the test object so as to enter into the view field.
Furthermore, the laser beam, entering into the EO sampling optical system from the pulse laser source 21 through the optical fiber, is reflected at the side of the EO crystal facing to the wiring. The reflected light, then reflectively bent at a right angle by the dichroic mirror 29, and again bent at an right angle by the half mirror 23, is verified by the operator from the images of the infrared camera 27, and the operator moves the XY stage such that the laser beam is condensed on a point on the wiring surface to be tested. At this time, the laser beam can be verified by the infrared camera 27 because the dichroic mirror 29 has a reflectance of 5% for light having the wavelength of the laser light.
Next, a test operation for measuring the test signal by use of an apparatus of the EO sampling measurement is described.
When a voltage is applied to the wiring on a test object, the phenomenon occurs that, due to the Pockels effect, the index of the double refraction of the EO crystal changes depending on the electric field applied to the EO crystal.
Thereby, when the laser light is entered into the EO crystal and propagates through the EO crystal, the polarization state of the laser light changes. The laser beam, after being subjected to the change of its polarization state, is reflected at the EO crystal surface facing to the wiring, and again impinges on the EO sampling optical system 22.
The light beam impinging on the EO sampling optical system 22 is isolated by a light isolator (not shown) in the EO sampling optical system 22. The isolated light is received by a diode (not shown), which converts it into an electric signal. The electric signal is amplified by the differential amplifier 24 and is then displayed on the waveform indicator 25, so that the measurement of the electric signal applied to the wiring on the test object 10 can be executed.
The principle of the EO sampling is shown in FIG. 5. When a signal is applied on the EO crystal, the change of the electric field in the crystal by the applied signal causes a change of the index of the double diffraction of the EO crystal due to the Pockels effect. When a laser beam passes through the thus changed crystal, the polarization state of the laser light changes according to the change of the double refractive index, so that it becomes possible to measure the change of the electric field, that is, the change of the signal, by measuring the change of the polarization state of the laser light.
In order to execute the above described measurement, it is necessary to place the EO crystal on the test object. Conventionally, an operator has placed the EO crystal 1 on the test objects 10, such as a semiconductor wafer or the like, using tweezers. However, the problem arises that the placement must be executed very carefully so as not to damage the test object, so that the conventional measurement has been difficult and can be successfully executed only by experienced operators.
There is a conventional method reported in a paper entitled xe2x80x9cAn on-wafer EO probing system for internal diagnosis of the ultra high speed ICxe2x80x9d (C-309, Proceeding of Electronic Information Communication Society, 1992) The constitution of the probing system reported in the above paper is shown in FIGS. 6 and 7.
FIG. 6 is a perspective view of the whole system, and FIG. 7 is a cross-sectional view of the system shown in FIG. 6.
In FIGS. 6 and 7, the reference numeral 10 denotes a test object such as a silicon wafer. The numeral 51 denotes an EO crystal. The numeral 52 denotes an objective lens, 53 a cylinder, 54 an air guide, and 55 a linear actuator. The numeral 56 denotes a position scale, 57 a balance mechanism, and 58 denotes a chuck for fixing the test object.
This system is a mechanism comprising the linear actuator 55 (a piezoelectric element), a balance mechanism 57, an air guide 54, a position scale 56, etc, and this system is added to an apparatus shown in FIG. 4 which allows automatic alignment of the EO crystal on the test object.
The measurement using the above mentioned mechanism is implemented by the steps of, first aligning the test object at a measuring location, and then the EO crystal 51 mounted at the bottom of a cylinder 53 is lowered until the EO crystal contact the test object. Since the combination of the EO crystal and the cylinder is integrated and heavy, the effective mass of the combination is reduced by the balance mechanism 57 in order to prevent an excess force from being applied to the test object. The contact height of the EO crystal with the test object is detected by the position scale 56, and the measurement is executed after raising the EO crystal by 1 xcexcm to the contact surface in order to maintain a narrow space between the EO crystal and the test object.
The above mechanism has high alignment accuracy. However, problems arise in that it takes the considerable time of 5 to 10 seconds because complicated and fine aligning is required, and in that the apparatus is complicated and expensive.
The present invention is made in order to solve the above mentioned problems. Therefore, the objective of the present invention is to provide a electro-optic sampling probe, which is simple in structure and which can be operated by an operator without requiring extensive training.
The electro-optic sampling probe according to the present invention is used for observing a waveform of a test signal based on the change of the polarization state of a light pulse caused when the light pulse generated by the timing signal is admitted into an electro-optic crystal which is coupled with an electric field generated by the test signal.
According to the first aspect of the present invention, the electro-optic sampling probe comprises an electro-optic crystal having a stopper portion at its upper portion; an electro-optic crystal supporting portion for supporting said electro-optic crystal by said stopper so as not to fall and capable of moving said electro-optic crystal up and down; and an electro-optic crystal driving portion, to which said electro-optic crystal supporting portion is attached, for rectilinearly moving said electro-optic crystal along the direction of an optical axis.
According to the second aspect, said electro-optic crystal driving portion is moved rectilinearly by a driving lever provided with a fulcrum supported at an observing optical system body.
According to the third aspect, said electro-optic crystal driving portion is moved rectilinearly by a rack and a pinion.
According to the fourth aspect, said electro-optic crystal driving portion is moved rectilinearly by a motor and a feed screw.
According to the fifth aspect, said electro-optic crystal driving portion is moved rectilinearly by an eccentric cam.